In conventional printing processes, printing presses that utilize offset plates, or other ink-transfer mechanisms, print a large block of book pages on a single sheet or "signature." FIG. 1 details a conventional printing process in which a pair of printing presses 20 and 22 are operated in tandem. Each press produces multi-page signatures 24 and 26 that, in this example, each include eight individual pages 28. The multi-page signatures are directed to respective stacks 30 and 32. Each signature 24 and 26 represents a discrete set of pages in a book. Typically, the stacks 30 and 32 are composed of a series of alike signatures since there is substantial set-up time and effort associated with changing the presses to prepare a new set of signatures. In other words, all signatures of a single group of pages are printed before the process moves on to the next set of signatures. Usually, a multiplicity of stacks of signatures must be prepared to create a finished book. For example, in an eighty-page book, ten separate stacks of signatures must be prepared. Each stack includes a total number of signatures that equals the number of books to be completed. Note that each signature can be reprinted on an opposing side so that the resulting pages include double-sided print.
After the printing step, the signature stacks 30 and 32 are directed to a cutter 34 in a selected order to produce a resulting stack 36 of bindable facing pages. The facing pages each consists of two individual page halves 28 that are folded along a center line 38 to generate the actual pages of a book. The stack 36 is ordered so that the facing pages can be folded and bound in to completed books 40 in a binding step. As described below, this stacking step is typically completed only after all signatures necessary to complete the book have been printed, cut and sorted. Appropriate binding equipment (not shown) is used to create completed, bound books. Covers and other decoration can be applied in later steps to the finished bound books in the stack 40.
A disadvantage of conventional printing processes is that groups of signatures must be collected, sorted and ordered before the cutting and binding steps can proceed. Unless a large number of printing presses are run in tandem to produce all the signatures necessary for the completion of a book at one time, several print runs through the same printing presses are required to create all the necessary signatures. Hence, stacks of signatures are typically stored awaiting completion of the printing run, and only then are the signature stacks fed in an appropriate order to cutters and binders. Often, the printing, storage, cutting and binding operations take place in different parts of the floor space of a printing house or even in different buildings. The multiplicity of dashed-lines 50 represent different points in a conventional printing process where substantial delays for storage and transport of print media awaiting completion typically occur. This conventional approach to book printing, thus, involves significant costs due to inherent delays in the process. In addition, the conventional printing approach entails additional costs for transport of materials around the production floor and for additional storage space.
The versatility of modem image transfer devices, such as high-volume laser printers, has become widely recognized in the printing field. Computer-driven, electronic print engines, such as laser printers, enable the user to instantaneously change the nature and quantity of the printing at any time during a production run. Electronic printing devices are versatile--able to print on single sheets, single-width webs, or double-width webs that are slit and merged at a later time. These printers can be arranged to print in duplex mode so that both sides of a sheet or web are provided with printing. A variety of cutters can be provided to reduce the size of sheets and/or webs to generate output that is similar to the stack 36 of double-faced sheets of FIG. 1.
However, forming a multiplicity of completed stacks of pages in book form directly from the printer poses certain problems. To enable transport, and to save space, books may be stacked. It is desirable that each book in the stack be justified along its edges to enable rapid binding. However, the separation of a large stack of completed books is problematic. Most conventional stack-separation techniques cannot ensure that the separated books will retain the desired justification. This may slow the otherwise quick and versatile electronic printing process in which complete books are produced in succession.
Given the versatility of computer driven print engines, it is an object of this invention to provide a print-on-demand process in which an entire book of sheets is generated in a single print run and such books are stacked in succession, free of the intermediate storage steps of conventional printing. It is the further object of this invention to provide an efficient technique for dividing stacks of finished sheets into individual books without misalignment of pages or undesirable adhesion of the pages of different books to each other.